Field of the Invention
The present invention relates to compositions comprising molecules with a pyrrole ring bound to a diol, and carbon allotropes.
In particular, the invention relates to adducts consisting of derivatives of serinol pyrrole and of carbon allotropes in which the carbon is sp2 hybridized, such as carbon nanotubes, graphene or nano-graphites, carbon black, in order to improve the chemical-physical properties of these allotropes increasing above all their dispersibility and stability in liquid media and in polymer matrices.
Description of the Related Art
Currently, one of the research subjects of greatest interest in the field of composite materials concerns the preparation of innovative materials from renewable sources. Key objectives of this activity are: the use of raw materials that do not have an impact on the food cycle both because they do not detract edible raw materials and because they do not use farmland, the implementation of synthesis processes with a low environmental impact both with regard to energy and because they do not use solvents and do not produce waste chemical substances, and the production of composite materials that do not cause problems of disposal after their use.
Among the raw materials from renewable sources, glycerol is of particular interest, as it has no impact on the food cycle, is non-toxic, biodegradable, readily available and low cost. In fact, glycerol is the main by-product of biodiesel synthesis. In 2011, 65% of glycerol was obtained through this process and a total amount of around 1.2 million tonnes was available on the market. Glycerol is considered the main “building block” for the development of an alternative C3 platform to the one deriving from petroleum. Derivatives of glycerol such as serinol are gaining increasing interest, both because glycerol is a pure chemical substance that can be obtained directly from renewable sources and for the chemoselectivity due to the presence of the amine group and of two hydroxyl groups that allow the design and implementation of different synthesis platforms.
It is known that carbon exists in various allotropic forms. Classification of the allotropes can be implemented based on hybridization of the carbon atoms of which this allotrope is formed. In the diamond, the carbon atoms are sp3 hybridized. In other allotropes, the carbon atoms are sp2 hybridized. These are: fullerene, graphene, graphite, carbon nanotubes, carbon black. Fullerene has the form of a hollow sphere, with 12 pentagonal faces and a varying number of hexagonal faces. Graphene is a layer of carbon atoms and therefore has the thickness of one carbon atom. Graphite, carbon nanotubes and carbon black are composed of graphene layers.
Graphite is composed of a variable number of graphene layers stacked in crystalline aggregates, with a typical distance of around 0.34 nm. The number of stacked layers may be less than ten and may reach several thousands. Carbon nanotubes can be viewed as formed of rolled graphene layers. One layer forms the single-wall nanotube, several layers form multiple or multiwall nanotubes. In each of these allotropes, cycles are present as base unit. As mentioned already for fullerene, these cycles can have 5 or 6 carbon atoms, each of which is involved in the formation of three sigma bonds and one π perpendicular to the plane on which the sigma bonds lie. The electrons involved in the π orbital are all delocalized on the aromatic polycyclic system. This is possible because the cycles are all condensed and form a single system. The simplest examples of aromatic polycondensation system are aromatic polycycles (including: pyrene, phenanthrene, anthracene). Fullerene, graphene and carbon nanotubes form the equivalent of aromatic polycondensation system with different degree of planarity. In order for a system to be defined aromatic three conditions must exist: (a) the system must be cyclic, (b) the atoms, involved in the cycle, must all have sp2 hybridization and the sum of the π electrons must satisfy Huckel's rule (π=4n+2, where n is an integer including zero), (c) the system must be planar. In the case of graphene, the requirements are all met. In the case of fullerene and of carbon nanotubes the curvature influences the condition of planarity. These systems can still be defined aromatic and represent an exception.
Carbon allotropes, in which carbon is sp2 hybridized, such as carbon nanotubes, graphene, graphite and carbon black, have electrical and thermal conductivity. In particular, carbon nanotubes and graphene have exceptional mechanical and electrical and thermal conductivity properties. In particular, they are capable of conducting electrons without dissipating energy in the form of heat. Moreover, they have nanometric dimensions, that is, they are smaller than 100 nm: one dimension, in the case of graphene, and two dimensions, in the case of nanotubes. This means they have a large surface area and are therefore capable of establishing a large interfacial area with the matrix in which they are located, greatly influencing its properties. As indicated above, graphite is formed of crystalline aggregates, in turn formed of stacked graphene layers. When the number of stacked graphene layers is low, from less than ten to a few tens, the dimension of the crystalline aggregate in the direction orthogonal to the layers ranges from a few nm to a few tens of nanometers. These graphites are called nano-graphites.
In fact, carbon allotropes can be divided into “nano” and “nano-structured”. Carbon allotropes such as fullerene, carbon nano-tubes, graphene and nano-graphites are “nano” allotropes. Carbon black, which has been used for over a century to reinforce elastomeric compounds, is instead “nano-structured”. A pure chemical substance is defined “nano” when it has at least one dimension of less than 100 nm. Fullerene, carbon nanotubes, graphene, nano-graphite and their derivatives containing functional groups of different nature and in different amounts are characterized by having at least one dimension of less than 100 nm. Graphene is a layer of sp2 hybridized carbon atoms, has the thickness of a carbon atom and therefore has nanometric dimension. Nano-graphites also have nanometric dimension, provided that the number of stacked graphene layers do not lead to a thickness of over 100 nm. Carbon nanotubes have two nanometric dimensions. Carbon black used as reinforcing filler consists of elementary particles, which have nanometric dimensions, combined to form aggregates in which these elementary particles are held together by covalent bonds. The thermomechanical stresses typical of the action of mixing of the carbon black with elastomeric matrices and also of the use of these matrices are unable to separate the aggregates into elementary components. Aggregation leads to the creation of empty spaces between elementary particles, creating a particular structure for the carbon black. The larger the number of empty spaces, the larger the structure is. This gives rise to the definition of nanostructured filler. Carbon black aggregates have dimensions greater than 100 nm. The aggregates then combine through van der Waals forces to create agglomerates, which can however be separated into the initial aggregates through thermal-mechanical stresses.
Due to their properties, carbon allotropes such as carbon nano-tubes, graphene and nano-graphites and carbon black are used both in polymer, plastic and elastomeric matrices and in coating layers. They promote mechanical reinforcement and thermal and electrical conductivity of the materials in which they are found. Improvement of the aforesaid properties is particularly marked when “nano” carbon allotropes, such as carbon nano-tubes, graphene and nano-graphites are used. Moreover, carbon allotropes such as carbon nano-tubes, graphene and nano-graphites in polymer matrices have a noteworthy flame retardant effect. In the case of polymer matrices, carbon allotropes can be mixed directly in these matrices, forming the final product through conventional mixing technologies, or can be part of predispersions, typically in concentrations greater than those used in the final product. Likewise, in the case of dispersions in liquid media, carbon allotropes can be part of the final formulation, to be used, for example, to form coating layers, or can be in a “masterbatch dispersion” to be used for the preparation of various formulations.
In the case of composite polymer materials containing carbon allotropes, an attempt is made to obtain optimal distribution and dispersion of the allotropes and above all to produce optimal interaction of the allotropes with the matrix and stable interaction in the conditions of use of the material. In the case of dispersions in liquid media, an attempt is made above all to obtain stability of this dispersion, preventing decantation of the allotrope. In fact, the greatest problem that can occur in the case of polymer composite materials containing carbon allotropes is insufficient interaction of the allotropes with the polymer matrix. This problem has been found in particular for “nano” carbon allotropes, such as carbon nano-tubes, graphene, nano-graphites. This leads to insufficient transfer of the properties of the allotropes to the composite material and leads to instability of the dispersion of these allotropes, which tend to aggregate, with considerable worsening of the properties of the final material. The greatest problem that can occur in the case of dispersions of carbon allotropes in both polar and apolar media, consists in the fact that these dispersions are not sufficiently stable to be used in industry, as the carbon allotropes tend to sediment. This problem has been found in particular for “nano” carbon allotropes, such as carbon nano-tubes, graphene, nano-graphites. The polar media can be low viscosity liquids such as solvents of normal use, in particular environmental friendly solvents, such as water, alcohols, ketones and esters. Examples of alcohols are ethanol and isopropanol, examples of ketones are acetone and methyl ethyl ketone, an example of ester is ethyl acetate, an example of amide is N-methylpyrrolidone, or can also be low viscosity liquids such as solvents of normal use, specifically those that are environmentally friendly, such as water, alcohols, ketones and esters.
Moreover, the polar media can be polymers, both amorphous and semi-crystalline. These polymers can have a group of polar nature in one or in all the repetitive units. Examples of polymers with a polar group in each repetitive unit are, for example: polyurethanes, polyethers, polyesters, polycarbonates, poly(vinyl esters), poly(vinyl alcohol). Examples of polymers that do not contain a polar group in each repetitive unit are, for example: copolymers of ethylene with polar monomers such as vinyl acetate, vinyl(alcohol). Other examples of polymers that do not contain a polar group in each repetitive unit are polymers in which the polar group has been introduced by means of grafting reaction. Examples of these polymers on which the grafting reaction can be obtained are polyolefins, such as poly(ethylene) and the poly(propylene), ethylene-propylene copolymers, polymers deriving from dienes, on which an anhydride such as maleic anhydride or itaconic anhydride have been grafted, or on which an ester such as ethyl maleate has been grafted, or on which a mixture of an anhydride and an ester has been grafted. There are also polymers that have apolar nature, but contain polar groups as chain terminals, such as natural rubber, i.e. poly(1,4-cis-isoprene) deriving from the plant hevea brasiliensis. 
“Carbon nano tube-polymer interactions in nanocomposites: A review, Composites Science and Technology 72 (2011) 72-84” describes carbon nano-tube based composites. Graphene based composites and nano-graphites are described in “Graphene-based polymer nanocomposites.” Polymer, 52(1), 5-25 (2011)”. In these two cases, carbon allotropes are used to prepare composites both in polar polymers such as polyacrylates and epoxy resins and in apolar polymers such as poly(ethylene) and poly(styrene). Dispersions of carbon nano-tubes in elastomeric matrices are described in “Multiwall carbon nanotube elastomeric composites: a review” Polymer, 48(17), 4907-4920 (2007) and in “The Role of CNTs in Promoting Hybrid. Filler Networking and Synergism with Carbon Black in the Mechanical Behavior of Filled Polyisoprene” Macromol. Mater. Eng., 298, 241-251 (2012). Dispersions of nano-graphites are also reported in elastomeric matrices, for example in “Filler Networking Of A Nanographite With A High Shape Anisotropy And Synergism With Carbon Black In Poly(1,4-Cis-Isoprene)—Based Nanocomposites” Rubber Chemistry and Technology, Vol. 87, No. 2, pp. 197-218 (2014). However, all these composites show carbon allotropes dispersed at the level of the single particles of which they are formed, that is, at the level of the single nanotubes or single graphene lamellae or aggregates with only a few graphene layers, but also show agglomerates. In particular, “Filler Networking Of A Nanographite With A High Shape Anisotropy And Synergism With Carbon Black In Poly(1,4-Cis-Isoprene)—Based Nanocomposites” Rubber Chemistry and Technology, Vol. 87,-); 2, pp. 197-218 (2014) shows how the nano-graphite aggregates tend to aggregate further, that is, to be composed of several graphene layers, when they are in the cross-linked elastomeric composite.
It is known that elastomers cannot be used for practical applications unless they are vulcanized and are reinforced through the addition of reinforcing fillers. For over a century carbon black has been the carbon allotrope of reference for reinforcing elastomers. To be able to perform the reinforcing action of an elastomeric matrix, a filler must not be soluble in the polymer matrix and must have a modulus significantly higher than that of this matrix and have sub-micrometric dimension above all of the particles of which it is formed and, preferably, also of the aggregates of these particles. In fact, the smaller this dimension, the larger the surface area, which means interfacial area with the polymer matrix. In fact, the interfacial area is given by the product of the properties of the filler such as surface area, density and fraction in volume. An extensive interface and good interaction between the reinforcing filler and the polymeric chains are therefore prerequisites for mechanical reinforcement, as they allow stress transfer to the polymer matrix, capable of storing energy. It is therefore evident how “nano-fillers” have great potential, due to their intrinsic modulus, to their nano-size and consequent high surface area. Moreover, it is known how the surface area is responsible for mechanical reinforcement with low strains. In fact, a high surface area promotes extensive interaction, which however could be due only to van der Waals forces, thus promoting low strain mechanical reinforcement, which is eliminated as this strain increases. The force applied to increase the strain eliminates van der Waals interactions between the filler and the polymer matrix. The high strain reinforcement is due to stable interaction between the polymer matrix and the filler. The structure of the filler, that is, the voids between the elementary particles of this filler, play a fundamental role in promoting this reinforcement. These voids receive the elastomer, which is immobilized and, so as to speak, itself transformed into filler. In the case of carbon black, “nano-structured” filler, in the presence of a smaller surface area there is less low strain mechanical reinforcement, whereas in the presence of a high structure (and many carbon blacks have a high structure) high strain mechanical reinforcement occurs. Therefore, both “nano” and “nano-structured” carbon allotropes have the prerequisites to perform an important mechanical reinforcing action of the elastomeric matrices. The prior art teaches that the surface tension of the reinforcing filler and of the polymer that forms the matrix cannot be too different in order to obtain effective interaction.
To produce an effective reinforcing action, the fillers must be used in considerable amounts. Typically, in standard ASTM compounds more than 30 parts of filler per 100 parts of elastomer are used. With this amount of filler, the filler is over its percolation threshold, and therefore forms a network. This generates energy dissipation mechanisms essentially due to the absence of weak interactions between filler aggregates, that is, the absence of the network, following the application of static and dynamic mechanical stresses. It is known how the elastic modulus of a filled composite material, to which sinusoidal stresses have been applied, decreases, passing from minimum strain up to around 25% of strain (limit estimated for linear behavior). This phenomenon is known as the “Payne Effect”, and is an indicator of the energy dissipation of the material. Decrease of the Payne effect, that is, of energy dissipation in a composite material, passes through the optimization of dispersion of the carbon allotrope, separating them to the smallest individual unit that can be obtained. In order to obtain stable dispersion both in liquid dispersion media with medium-low viscosity and in polymers, the carbon allotropes must be modified both through chemical modifications that lead to the formation of covalent bonds with functional groups, producing functionalizations of the allotropes, and through noncovalent chemical modifications, that is, supramolecular interactions.
WO2010/102763 describes semi-crystalline polyurethane compositions in which carbon nanotubes are dispersed in order to improve their properties. In this case the modifications take place through the use of polymer chains grafted to the carbon allotrope that allow dispersion in polyurethane. However, in this case interaction between the polyurethane and the allotrope is not stable as it occurs only due to carbon group present in the polymer. In the absence of a stable interaction, the carbon nanotube dispersed in the polymer matrix, or in a liquid medium, tends to sediment and to separate from the medium, creating areas with a higher concentration of nanotubes and areas with no nanotubes, consequently changing the properties of the product.
US2006/0045838 describes adducts between carbon nanotubes and soluble polymers selected from poly(thiophene), poly(pyrrole), poly(fluorene), poly(phenylene), poly(phenylene ethynylene), poly(phenylene vinylene), poly(alkylidene fluorene), poly(fluorenebithiophene) and combinations thereof. Also in this case, the modifier is of polymer nature. The nature of the polymers is clearly lipophilic and this implies the choice of organic solvents such as chloroform for their dissolution, solvents that have criticalities from the point of view of impact on the environment and on health. Moreover, these adducts are unable to provide stable dispersions in polar solvents with low environmental impact, such as aqueous solvents. Moreover, the lack of stability of these adducts leads to non-homogeneous dispersion of the nanotubes.
The possibility of dispersing carbon allotropes in aqueous solvents is also known. Surfactants such as sodium dodecyl sulfate are used, as reported in “SDS Surfactants on Carbon Nanotubes: Aggregate Morphology” ACS Nano, 2009, 3 (3), pp. 595-602. In this case, advantage is drawn from the interaction between the dodecyl substituent and the allotrope, while the salt ensures dispersion in water. “Decoration of carbon nanotubes with chitosan” Carbon, 43(15), 3178-3180 (2005) shows the dispersion of carbon nanotubes in acid solutions (pH =5) preparing the adduct of the carbon nanotubes with chitosan. In this case, interaction between the ammonium cations and the it systems of the nanotubes is exploited. It is evident how these modifiers reduce the properties of the allotropes, not contributing to any extent to the electrical and thermal conductivity of these allotropes.
The possibility of solubilizing a polymer with aromatic monomer in an aqueous environment is also known. For example, a water soluble polymer of a pyrrole substitute is obtained by means of electro-oxidative polymerization of potassium 3-(3-alkylpyrrol-1-yl)propanesulfonates, as reported in “Lamellar Conjugated Polymers by Electrochemical Polymerization of Heteroarene-Containing Surfactants: Potassium 3-(3-Alkylpyrrol-1-yl) propanesulfonates” Chem. Mater. 1994,6, 850-851.
A water soluble polypyrrole is reported in “A Water-Soluble and Self-Doped Conducting Polypyrrole Graft Copolymer”, Macromolecules 2005, 38, 1044-1047. A poly copolymer (sodium styrenesulfonate-co-pyrrolylmethylstyrene) is used as precursor for polymerization of the pyrrole contained as side group in the polymer with other units of non-substituted pyrrole.
In these two examples, synthesis of a substituted pyrrole or of a polymer containing a pyrrole ring is necessary. The yields of these reactions are not high and are not conducted using ingredients from renewable sources. Otherwise, the post treatment of polypyrroles is reported in “Synthesis and characterization of water soluble polypyrrole doped with functional dopants” Synthetic Metals 143 289-294 (2004). Sulfonation of a polypyrrole is performed. In this case, it is not possible to obtain a polymer containing aromatic rings such as that of pyrrole and polar groups directly through polymerization.
It would be desirable to be able to prepare stable dispersions of carbon allotropes both in liquid media and in polymer matrices, producing adducts of carbon allotropes with compounds that contain functional groups capable of interacting with the aromatic rings of the carbon allotropes, consequently groups containing π electrons such as aromatic or carbonyl rings, or ammonium groups, or also only lipophilic groups, without however compromising the possibility of dispersing the adducts in matrices and in polar solvents. In particular, it would be desirable to be able to use solvents with low environmental impact such as alcohols, ethers, esters and even aqueous solvents.
It would be desirable to obtain compounds that comprise both the functional group that promotes interaction with the carbon allotrope and other functional groups. That is, it would be desirable to produce compounds from the molecule containing the functional group capable of interacting with the carbon allotrope.
It would also be desirable to be able to obtain a synergy between the functional groups capable of interacting with carbon allotropes, being able, for example, to combine aromatic rings and other functional groups containing π electrons, such as carbonyls.
It would be desirable for the modifying agents used to prepare the adducts not to reduce the properties of carbon allotropes. In particular, it would be highly desirable for the modifying agents to contribute to the electrical conductivity.
Moreover, it would be desirable for the stable adducts of polymers with carbon allotropes to be easily achievable. In particular, it would be desirable to be able to use simple synthesis and preparation techniques. It would also be desirable to be able to used different preparation methods.
It would be desirable for the structures used to allow stable dispersions of carbon allotropes to be obtained, in order to maintain their properties over time.
Therefore, it would be desirable for the stable dispersions of allotropes in liquid media or in polymer matrices to be easy to produce.
It would be desirable for the compounds capable of interacting stably with carbon allotropes such as nanotubes, graphene and nano-graphites to be produced from renewable sources, which preferably have no impact on the food cycle, so as to obtain a low environmental impact, in terms of energy required for preparation, both because they do not use solvents and do not produce discarded chemical substances, and in terms of disposal of the materials after their use.
An object of the present invention is therefore to provide stable adducts between a carbon allotrope in which the carbon is sp2 hybridized and a compound containing functional groups capable of interacting with the aromatic rings of carbon allotropes.
Yet another object of the present invention is to provide compositions that are easy to obtain, produced from renewable and natural sources that have no impact on the food cycle so as to obtain materials with a low environmental impact both in terms of energy linked to their preparation and in terms of pollution caused by their disposal.
Moreover, an object of the present invention is to provide compounds capable of interacting with carbon allotropes in a stable and efficient manner and that do not compromise the possibility of dispersing carbon allotropes also in polar solvents, even water-based.
Moreover, an object of the present invention is to provide structures capable of interacting with carbon allotropes in a stable and efficient manner and that can, to some extent, contribute to the properties of the carbon allotrope, such as electrical conductivity.